Enhanced formalin nociceptive responses following L5 nerve ligation in the rat reveals neuropathy-induced inflammatory hyperalgesia.

The development of mechanical and thermal hypersensitivity following peripheral nerve injury is well known and a great deal of research has been directed towards understanding the mechanisms underlying these phenomena. However, there has been very little research examining if hypersensitivity to an inflammatory condition following nerve injury also develops. Therefore, the purpose of the present study was to determine if hypersensitivity to an inflammatory condition produced in the formalin test develops following ligation of the L5 spinal nerve. Male Sprague-Dawley rats received tight ligation of the L5 spinal nerve or were given sham surgery. Following a 14-day recovery period, the threshold to produce a withdrawal response to a mechanical stimulus was measured using von Frey monofilaments and then formalin behavioral responses were measured. Compared to sham animals, L5 ligated animals exhibited significantly lower mechanical paw withdrawal thresholds as well as elevated and prolonged nociceptive responses during the second phase (20-60 min) of the formalin test. These results reveal enhanced inflammatory nociceptive processes following peripheral nerve damage and might provide a useful approach to study underlying neural mechanisms associated with clinical neuropathic pain syndromes.